Physiology of Finfish and Shellfish (2+1)

2.1.27. Accessory respiratory organs in fishes

Accessory respiratory organs in fishes are generally found in the fishes which live in shallow stagnant freshwater of tropical regions where deoxygenation of water is a main feature. Such structures may also develop in fishes inhabiting torrential streams of the hills, which are liable to dry up during summer.

1. Buccopharyngeal epithelium: This is the most primitive type of adaptation and has been found in Monopterus javanensis, Electrophorus electricus, Periopthalamus sp and Bleopthalamus sp. In these fishes, the epithelial lining of the mouth cavity is highly vascular and forms vascular network and thus work as efficient respiratory site.
2. Skin: The extent to which fishes use cutaneous respiration is poorly known. It is best documented in eels (Anguilla anguilla), which leave the water and migrate from one place to another through damp vegetation. During this time, the moist skin (richly supplied with blood) work as an important organ for aerial respiration. In some fishes like Monopterus and Mastacembelus, which live in oxygen deficient stagnant waters, the skin is of little use for respiration. The glandular secretions of the skin protect skin from desiccation in water deficient condition.
3. External gills: These are highly vascular and filamentous structure of the ectoderm covering the outer surface of the visceral arches or visceral skeleton. They are in direct contact with water, so they help in exchange of gases. To aid respiration, the embryos of certain elasmobranchs possess long filamentous structures present as outgrowth from spiracles. (External gills - develop from the skin ectoderm of the branchial area but are not directly related to the visceral skeleton or branchial chambers are found most often in larval or paedomorphic amphibians)
4. Labyrinthiform organs: These organs have been present in few perches like Anabas testudineus and Trichogaster fasciatus. These are formed within the extrabranchial chambers enclosed between the gills and the operculum. These organs are originated from the 1^st gill arch.
5. Opercular lungs: In advanced air breathing teleosts, the bag like diverticulae is called opercular lungs or air chambers. It develops from the dorsal surface of the branchial chamber or opercular chamber. The opercular lungs are present above the gills and contains specialized structures known as rosettes or arborescent organs (arbo = tree) which just increase the respiratory surface. Such accessory respiratory organs are found in Clarias, H. fossilis, A. testudineus, T. fasciatus etc.
6. Air bladder/ swim bladder: Swim bladder is present between the gut and kidney. This duct degenerates in the course of development or present throughout the life. The fishes with air bladder have adapted to extensive drought conditions which may completely dry up. The teleosts like Chirocentrus, Cyprinoids come to the surface take air and pass it back to the air bladder which is highly vascular. (Swim bladder is a gas filled bag like structure which originates from the dorsal outgrowth from the alimentary canal in most of the ray finned fishes. It controls buoyancy at different depths in water bodies. The gases which are present in the air bladder are oxygen, carbon dioxide and nitrogen).
7. A special part of alimentary canal: In a small group of fishes, inhaled air is swallowed and forced back inside the alimentary canal, a part of which may be modified for aerial respiration. After respiratory exchange, the used up air is either passed outside by the anus or is expelled through the mouth. For helping the gaseous exchange some modifications have taken place in alimentary canal. There is a great reduction in muscle layers of stomach and intestine making it thinner and transparent. Inner surface is lined by a single layer of epithelial cell which is highly supplied with blood.